1. Field of the Invention
The invention relates to optical components, and more particularly to a low color shift polarizer assembly, and a back light unit and a liquid crystal display containing the color shift polarizer assembly.
2. Description of the Related Art
Liquid crystal displays (LCD) operate by utilizing a light valve mechanism comprising a polarizing light and a liquid crystal layer, wherein voltages are used to change retardations of the liquid crystal layer to display images. The polarizing lights normally operate by using polarizers to separate polarizing lights having single axes from natural lights of the light sources. However, at least a half of the incident lights are absorbed by the polarizers in the above described process, such that the efficiency of the light energy is reduced.
In order to overcome the above described problems, polarized states translation films are used to transfer natural light without being polarized into a single polarized state light. In this example, brightness of the liquid crystal display can be enhanced, since a polarized axis of the polarized states translation film corresponds with a transmissive axis of the polarizer. Meanwhile, U.S. Pat. No. 5,235,443 discloses a method for increasing brightness of projection displays, wherein a cholesteric liquid crystal layer is coated on a spherical surface of a light bulb.
The cholesteric liquid crystals can selective reflect and transmit lights of specific wavelength ranges. The lights can be separated by polarization through selection of the polarizing lights of the cholesteric liquid crystals. The wavelength ranges which can produce polarization separation are dependant upon the sizes of the pitches and the reflective indexes of the cholesteric liquid crystals.
A helical pitch p of the cholesteric liquid crystals is represented by the liquid crystal molecules in the same layer having an average axis in a helical structure of the cholesteric liquid crystals, wherein the average axis is rotated an angle along a direction of the helical axis with one by one the layers. Note that the rotating angle is 360 degrees, and a distance in the direction of the helical axis is the helical pitch p. The helical pitch p is a character of the cholesteric liquid crystals. Thus, a polarizing reflective wavelength λ and a width of the wavelength Δλ of the cholesteric liquid crystals through the helical pitch p is expected, and can be represented by the formulas below:λ=n·p; andΔλ=Δn·p, 
wherein n=an average reflection of the cholesteric liquid crystals, Δn=a complex reflection (ne-no) of the cholesteric liquid crystals, and Δλ=the width of the polarizing reflective wavelength of the cholesteric liquid crystals. In a natural condition, the Δλproduced by Δn is about a width of several tens of nm. A cholesteric liquid crystal film of a large Δλ can be formed by different temperatures or through the fabrication methods of U.S. Pat. Nos. 6,669,999, 5,506,704 or 5,691,789.
A cholesteric liquid crystal film polarized element can be disposed between a backlight module and a display panel. In this example, when the lights from the backlight module are able to enter the cholesteric liquid crystal film, only the circular polarizing lights having a rotation direction contrary to a helix direction of the cholesteric liquid crystal film passes through the cholesteric liquid crystal film. Specifically, the circular polarizing lights having a rotation direction which is the same as a helix direction of the cholesteric liquid crystal film, is reflected toward the backlight module, recycled by the reflection mechanism of the backlight module and recombined with the backlight to be transferred into the display panel again. Therefore, brightness of the display is enhanced by about two times.
However, color shift in off-axis directions of the polarizing light separation film easily occurs in the cholesteric liquid crystal film. Referring to FIG. 1a, a cross section of a conventional cholesteric liquid crystal film 500 without grooves is shown. The liquid crystal molecules in the conventional cholesteric liquid crystal film 500 without grooves are normally arranged to form an area without grooves or a uniform domain, such that a significant mirror surface effect is produced. When lights enter the conventional cholesteric liquid crystal film 500 without grooves, transmission and reflections with simple and identical directions are produced. Therefore, better transmission of lights is produced and enhanced brightness effect and lower color shift are obtained in a small viewing angle range of the conventional cholesteric liquid crystal film 500 without grooves. However, along with an increase of the viewing angle, color shift with light color different to light color of the original light can be easily observed in an off-axis large viewing angle range. Referring to FIG. 1b, a schematic view of the light distribution states for the light S1 leaving the conventional cholesteric liquid crystal film 500 without grooves is shown. An angle between the path B of the light and the normal line L1 is larger than that between the path A of the light and the normal line L1, such that the exit light B3 from the light of path B has higher color shift. The lights observed at a detective point D1 of the large viewing angles consist of high color shift lights B3. Therefore, the color shift of the conventional cholesteric liquid crystal film 500 without grooves is not improved and the enhanced brightness effect at large viewing angles is significantly reduced. Specifically, the significant changing of vision effects with viewing angles are undesirable for display applications, especially for the applications of large size displays. For detailed discussion of the color shift, reference may be made to D. Coates, et. al. IDW'96 Proceeding p. 309.
U.S. Pat. No. 5,731,886 discloses a method for color shift compensation. A positive C-plate is provided for color shift compensation. However, color shift compensation by this method is limited and the selectivity of materials for the positive C-plates is limited. Moreover, it is difficult to directly fabricate a large size positive C-plate on a polarizing light separation film. U.S. Pat. Nos. 5,808,794, 6,449,092 and 6,088,159 disclose a method for extending an effective wavelength range of a polarizing light separation film toward an infrared ray area. However, the lights used for the resulting displays normally do not have continuous spectra, and the method is only applicable to some polarized light separating films with certain properties and some certain viewing directions. Therefore, the method still does not completely overcome the problem with color shift of displays at large viewing angles.
Therefore, a polarizer assembly is desired to overcome color shift of displays at large viewing angles and to achieve enhanced brightness effect.